1. Field of the Invention
This invention is related to a nonvolatile semiconductor device.
2. Description of the Related Art
As one type of nonvolatile semiconductor device, there is a NAND type flash memory. A NAND type flash memory is characterized by large capacity and low cost. Among flash memories, a multi-level NAND type flash memory is now receiving attention. Recently, the development of multi-level NAND type flash memories is progressing. A multi-level NAND type flash memory can store a plurality of data bits in one memory cell.
By using a multi-level NAND type flash memory, a memory system can become large in scale. The size of data which is programmed and/or read in such a memory system can become various. In order to accommodate various data, for example, as revealed in the US patent application publication US 2005/0218460 A1, there are memory systems arranged with flash memory devices which have different capacity or access speed. Alternatively, in the field of NAND type flash memory devices, multi-level type flash memories with many storage bits per memory cell are being developed due to an increase in the capacity of data storage. A device which can store two bits of data per memory cell, for example, is widely known as one type of a multi-level NAND type flash memory. The development of a device which can store four bits of data per memory cell is also progressing.
And, in the case of a multi-level NAND type flash memory, there are cases where programming and/or read speed becomes slower as the number of bits which can be stored per memory cell increases. In this case, the functions of a memory system which utilizes a multi-level NAND type flash memory are sometimes restricted.